How do I pass an argument to an operator that is called inside a panel?
General example based on your starting point including a property group setup.

import bpy
from bpy.types import PropertyGroup


class MyToolPropertyGroup(PropertyGroup):
    testint = bpy.props.IntProperty(  # bl 2.80 use testint: bpy.props
        name="testint",
        description="",
        default=1,
        min=1,
        )


def DO_SOMETHING(arg1, arg2):
    print("%i * %f = %f" % (arg1, arg2, (arg1*arg2)))


class SimpleOperator(bpy.types.Operator):
    """Tooltip"""
    bl_idname = "object.simple_operator"
    bl_label = "Simple Object Operator"

    @classmethod
    def poll(cls, context):
        return context.active_object is not None

    def execute(self, context):
        props = context.scene.MyPropertyGroup
        my_val = 2
        DO_SOMETHING(props.testint, my_val)
        return {'FINISHED'}


class MyPanel(bpy.types.Panel):
    bl_space_type = 'VIEW_3D'
    bl_region_type = 'TOOLS'
#    bl_category = "Tab Name"  # not used in blender 2.80
    bl_context = "objectmode"
    bl_label = "mypanel"
    bl_idname = "OBJECT_PT_MyPanel"

    def draw(self, context):
        layout = self.layout
        obj = context.object
        props = context.scene.MyPropertyGroup
        row = layout.row()
        row.prop(props, "testint")
        row = layout.row()
        row.operator('object.simple_operator')


classes = [SimpleOperator, MyPanel, MyToolPropertyGroup, ]


def register():
    for cls in classes:
        bpy.utils.register_class(cls)
    bpy.types.Scene.MyPropertyGroup = bpy.props.PointerProperty(
            type=MyToolPropertyGroup)


def unregister():
    for cls in classes:
        bpy.utils.unregister_class(cls)
    del bpy.types.Scene.MyPropertyGroup

if __name__ == "__main__":
    register()